1. Field of the Invention
This invention relates to a process for the solid phase polymerization of nylons and especially to the solid phase polymerization of the polymer of tetramethylene diamine and adipic acid, more simply referred to as nylon 4.6. Nylons treated in the process of the invention, as compared with prior art processes, combine improved properties of higher molecular weights, higher melt strengths, higher melt viscosities, less degradation and lower degradation ratios. The nylon resins of this invention are well suited for use in melt forming processes such as extruding, casting, blowing, spinning and other processes in which high melt strength and high melt viscosities are needed.
2. Definitions
As used herein the following terms and test procedures are defined as follows:
1. Melting point (MP). The exothermic peak which occurs during heating of small samples in a differential scanning calorimeter (DSC) (ASTM D3417). PA0 2. Glass transition temperature (T.sub.g). The damping peak which occurs between the hard glassy phase and the rubbery phase during heating of material on a dynamic mechanical analyzer (DMA) (ASTM 4065). PA0 3. Relative viscosity (RV). The relative viscosity compares the viscosity of a solution of polymer in formic acid with the viscosity of the formic acid itself (ASTM D-789). The test results reported in this specification were obtained using 10.98 grams of nylon 4.6 dissolved in 100 cc. of formic acid at 25.degree. C. PA0 4. Melt viscosity (MV). An indicator of the melt flow characteristic of a resin as measured in Pascal seconds (Pa.sec) with a Monsanto capillary melt rheometer measured at 316.degree. C. under constant pressure conditions. PA0 5. Degradation ratio (DR). A measure of the degree of degradation of the melt viscosity of a resin upon heating to above the melting point of the resin calculated by dividing the determined melt viscosity after 5 minutes dwell time by the melt viscosity after 17 minutes dwell time. PA0 6. Heat deflection temperature (HDT). The temperature at which a rectangular bar of regular cross section deflects 0.025 cm. under a load of 1820 kPa (264 psi) as specified in ASTM D 648-82. PA0 7. Particulate. An adjective used to describe resins in the form of discrete particles. Particulate resins are sometimes made by chopping small diameter (e.g. 0.5 cm) extruded rods into approximately 0.3 to 0.8 cm. lengths and are also made by compacting powders into small pellets, usually &lt;1 cm. in diameter. The exact size and shape of particulate resins (also sometimes referred to as pellets or molding resins) useful in this invention are not of great importance other than that, for convenience in handling, they should be larger than fine powders and for efficient and reasonable treatment times they should be smaller than about 2 cm., and preferably smaller than about 0.5 cm., in diameter. It should also be mentioned that reference to nylon 4.6 resins, whether in particulate form or otherwise, refers to resins having molecular weights above about 15,000, which are useful, at least, in injection molding processes. PA0 8. Reactive and non-reactive gases. "Reactive gas" is used to mean a gas that will react with nylon 4.6, as by oxidation, at the temperatures and conditions to which the nylon is exposed in the processes of the invention and "non-reactive gas" is used to mean gases that will not react with the nylon under these conditions. PA0 9. Extractable impurities. This term is used to mean polymerization residues, such as monomers and oligomers, which can be removed by extraction in water or cyclohexane or by devolatilization at temperatures below the melting point of the nylon. In the case of nylon 4.6, extractable impurities begin to volatilize at useful rates at a temperature of about 175.degree. C. PA0 10. Molecular weight. Unless otherwise indicated, all molecular weights are given as number average molecular weights.
3. Discussion of the Prior Art
Nylons are filling a growing need for polymeric materials that maintain strength and solvent resistance in elevated temperature environments, i.e. at temperatures which may sometimes approach 300.degree. C. Devices mounted in the engine compartments of cars and trucks are examples of this need in which the quest is never ending for lighter and stronger materials having good strength and chemical resistance. Films, monofiliments and fibers with good high temperature properties are also being sought.
Among the nylons, nylon 4.6 (nylons are identified in the specification and claims using "." [a period] to separate the number designation of amine and acid groups and "/" [a slash] to separate copolymer components) is particularly suitable for these purposes because of its high melting point (MP), high glass transition temperature (T.sub.g), and high heat deflection temperature (HDT), all of which thermally related properties exceed those of other commercially available aliphatic nylons. In the molecular weights in which it is polymerized and made available, however, nylon 4.6 is difficult to process in melt processes such as extrusion, film casting, film blowing, blow molding and fiber spinning since these processes require resins with high melt viscosities, high melt strengths and good melt stability (low degradation ratio). The melt properties of resins used in these processes are unlike those needed or desired in injection molding since high melt viscosities decrease flow and increase cycle times. Melt stability is usually of lesser concern in injection molding because of the characteristic short cycle times.
It is well recognized that all thermoplastic polymers show a change in properties with changing molecular weight. Linear nylons exhibit increasing ductility, toughness, melt viscosity and melt strength with increasing molecular weight. Since these properties are especially beneficial in extrusion, film forming, fiber spinning and blow molding processes, it follows that methods by which higher molecular- weight nylon resins can be obtained are of utility and commercial importance.
The required melt viscosity of resins used in melt forming processes varies considerably in practice depending upon the size and shape of the article formed, the forming process, and the residence time of the melted resin in the processing equipment. Since melt viscosities are usually unstable and degrade above the melting point of the resin, the residence time in the melt must be taken into account in determining the required properties of the resin; that is, the properties of the resins entering a melt forming process must be high enough to accommodate the thermal degradation imposed by the process.
It has been observed that nylon 4.6 molding resins are generally more sensitive to thermal degradation than are other of the common types of nylon particularly due to the high processing temperatures required. To quantify the expected thermal degradation (e.g., loss in melt viscosity) of resins when detained above their melting points, an arbitrary test has been devised which provides a numerical value called the "degradation ratio" as defined above. A degradation ratio near unity suggests a melt stable resin and higher ratios suggest resins with increasing susceptibility to degradation while in the melt.
Common criteria for the melt forming of nylons by extrusion, blowing, casting and spinning include melt viscosities in a range of from about 600 to 1,000 or more Pascal.seconds, relative viscosities in a range of about 140 to 200 or more, and degradation ratios of about 7 or less. The highest melt and relative viscosities are needed in the extrusion of articles having diameters or thicknesses exceeding about 2.5 cm (sometimes referred to as heavy section extrusion) because of the especially long times the resins are held above the melt. At the lower end of the scale, that is at melt viscosities of about 600 Pascal.seconds and relative viscosities of about 140, cast film can be extruded. The requirements of other extruding, blowing, and fiber spinning processes usually lie somewhere in between these values.
Numerous processes for increasing the molecular weight of nylons have been proposed in the prior art. These processes may be divided into those that are conducted above the melting point of the polymer and those that are conducted below the melting point of the polymer. It is the latter type of process with which this invention is concerned and is referred to as "solid phase" or sometimes "solid state" polymerization.
Solid phase polymerization via chain extension of a previously polymerized nylon is particularly useful when there is a need to increase the melt viscosity and melt strength of nylon molding resins. One reason this is true is that the equilibrium between nylon monomer and polymer favors polymer formation at lower temperatures. The lower temperatures used in solid phase polymerization processes are also of benefit since undesirable side reactions are less apt to occur and degradation is minimized.
The susceptibility of nylons to solid phase polymerization was recognized early in the development of nylon as is disclosed in U.S. Pat. No. 2,172,374. Here it is taught that the degree of polymerization can be increased by heating a polymer of a diamine-dibasic acid type at a temperature below its melting point but high enough to effect polymerization. However, attempts which have been made to increase the molecular weight of nylon 4.6 resins by using the teachings of U.S. Pat. No. 2,172,374 have not proved successful, possibly because the experimental results reported in the patent were obtained using "half-made", non-fiber-forming nylons.
The prior art has also recognized that superheated steam can be used to increase the molecular weight and melt viscosity of nylon. U.S. Pat. No. 3,420,804, after noting that there exists an intimate relationship between molecular weight and relative viscosity of nylons, teaches the treatment of nylons at temperatures below their melting points in an atmosphere of superheated steam. It is disclosed that the relative viscosity of nylons having initial values of between 20 to 50 can be increased in viscosity by a value of at least 10, i.e. to relative viscosities of from 30 to 60. The relative viscosities given in the patent are defined as the ratio of the viscosity of a solution of 10.98 grams of nylon in 100 cc. of ninety percent by weight formic acid at 25.degree. C. to the viscosity of the formic acid at the same temperature.
It has been reported that useful solid phase polymerization of nylon 4.6 can be achieved at elevated temperatures below its melting (circa 295.degree. C.) in a nitrogen atmosphere in which steam (water) is present. It was observed that if the process is conducted under anhydrous conditions, by-products and products of degradation are formed which cause an observable change in color and detract from the physical properties of the nylon 4.6.
Other examples of the solid state polymerization of nylon 4.6 in the presence of water (steam) can be found, for example, in U.S. Pat. Nos. 4,460,762 and 4,757,131. Each of these patents describe an in-line two step process in which a prepolymer of nylon 4.6 is polymerized in the melt to a relative low molecular weight, i.e. below about Mn 10,000, and then the polymerization is continued in an steam bearing nitrogen atmosphere at temperatures of above about 200.degree. C. to yield a polymer having a number average molecular weight of 15,000 and over. These patents use the phraseology "after condensation" which, in the context in which it is there used, has the same meaning as the phrase "solid phase polymerization" used in the present specification.
All the useful prior art processes which are known for the solid phase polymerization of nylon 4.6 require the presence of some water which it has been believed is necessary for reactions to proceed between the end groups of nylon chains, at least at meaningful rates and without excessive degradation as evidenced by an observable yellowing of the nylon.
The prior art processes for the solid phase polymerization of nylon 4.6 in the presence of superheated steam, as compared with the processes of the here disclosed invention, suffer from the disadvantage that relatively high temperatures are required to obtain the desired high melt viscosities at useful rates. For example, it is reported in Example III of U.S. Pat. No. 4,757,131 that a solid phase polymerization was conducted at 255.degree. C. in a flowing stream of nitrogen containing 25 wt % water vapor. The relative viscosity, as determined in a solution containing 1 gram in 100 ml of 96 wt % sulfuric acid, is given as 3.7 after a 19 hour period--which suggests that a number average molecular weight of approximately 25,000 was achieved.
Reference is also made to an article entitled "Preparation and Properties of High Mass Nylon-4,6: A New Development in Nylon Polymers" which appeared in PLASTICS, 1985, Vol 26, September co-authored by a co-inventor of the above cited U.S. Pat. No. 4,757,131. It is here reported (vide FIG. 4, page 1585) that a solid phase polymerization of a typical nylon 4.6 prepolymer conducted at 260.degree. C. for a period of 35 hours in a flowing stream of nitrogen containing 25 wt % water yielded a polymer with a relative viscosity (1 g/100 ml in 96% sulfuric acid at 20.degree. C.) of about 8--which equates to a number average molecular weight of about 50,000.
By way of distinction from the U.S. Pat. No. 4,757,131 and the article in PLASTICS, the examples that follow will demonstrate that in the practice of the instant invention, number average molecular weights of over 70,000 are attained in about 36 hours at 200.degree. C. whereas the patent reports attained molecular weights of about 25,000 (estimated from the 3.9 relative viscosity in sulfuric acid) in 19 hours at 255.degree. C. and the article reports attained molecular weights of about 50,000 (estimated from the 8 relative viscosity in sulfuric acid) at 260.degree. C. Considering the generally accepted approximation that the rate of a chemical reaction doubles with each incremental increase of 10.degree. C. in temperature, it can be appreciated that the results achieved in the practice of present invention are unexpected and represent a substantial advance in the art.
The prior art processes for the solid phase polymerization nylon 4.6 are also often unsatisfactory because of the unacceptable degradation ratios of the treated resins. This is reported, for example, in U.S. Pat. No. 5,064,700 at column 1, lines 35, et seq., wherein the prior art processes for the solid state polymerization of nylon 4.6 in nitrogen and water vapor are discussed. The patentees write that the molecular weight of resins treated in these processes decrease substantially during melt processing ". . . whereby it is difficult to form a melt having a satisfactory melt viscosity."